Volvic stone

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The volvic stone of light gray color is of volcanic origin. He found in the construction industry rich use, especially in the French Puy-de-Dôme around Clermont-Ferrand and Riom . It mainly comes from tunnels and quarries near Volvic .

geology

Clermont-Ferrand Cathedral, built of Volvic stone.

The Volvic stone, French pièrre de Volvic , also lave de Volvic or andesite de Volvic , is a trachyandesite from which the uppermost lava flow of the Puy de la Nugère is composed. The Puy de la Nugère went through a rather complex history of eruption in the period 14,000 to 10,000 years BP . In its last effusive stage, it secreted lava flows, including the trachyandesite of the Volvic stone.

description

The rock is a relatively low-phenocrystalline volcanic rock with a gray color. It has many small, irregularly shaped bubbles (vacuoles) and looks almost pumice-like . Phenocrystals that are easily recognizable with the eye are hornblende ( amphibole ) and plagioclase ( andesine ). Other rarer phenocrystals are clinopyroxene , iron-titanium oxide, apatite and zirconium . The microliths are olivine , alkali feldspar and pigeonite . Two generations of microliths can be distinguished under the microscope: hornblende and andesine as a coarser fraction, among which smaller microliths of oligoclase and pyroxene , themselves surrounded by light-colored glass, have mixed.

The stone is frost-resistant and resistant to chemicals. Its expansion coefficient is low and the rock is therefore well suited for building projects.

Geochemical composition

Geochemically , the Volvic stone can be classified as a potassium-bearing benmoreite , which has the following composition of its main elements:

Oxide
wt.%		 Volvic stone
K-Benmoreit
SiO 2 57.10
TiO 2 1.12
Al 2 O 3 17.89
Fe 2 O 3 6.83
FeO
MnO 0.19
MgO 1.94
CaO 4.53
Na 2 O 5.42
K 2 O 3.40
P 2 O 5 0.55
H 2 O 0.05

The Volvic stone is undersaturated with quartz and normatively does not contain nepheline , but olivine . The following normative components can be determined: or 20.42%, from 46.60%, an 14.69%, where 1.88%, di 1.90%, en 2.88, fs 4.32, ol 2, 36, mt 1.50%, il 2.16% and ap 1.33%. The rock is thus in the border area between the SiO 2 -undersaturated and the SiO 2 -saturated members of the alkaline fractionation series in the Chaîne des Puys - the undersaturated members due to normative nepheline and the oversaturated members due to the occurrence of tridymite or cristobalite and normative Olivine to be labeled.

Dating

In connection with an investigation of the Volvic stone, the last eruptive stage at Puy de la Nugère (stage 4) was dated using thermoluminescence on plagioclase. It resulted in 10,900 ± 1,200 years BP.

Dismantling

The Volvic stone comes (as the name suggests) from quarries in the western municipality of Volvic. The mining is likely to have a very long tradition. A high point in the quarrying of ashlar was undoubtedly associated with the construction of the cathedral of Clermont-Ferrand in the 13th century .

Initially, the stone was mined underground using tunnels. In the 19th century the transition to open-cast mining took place, which drastically increased the mining volume. During the 19th and 20th centuries , mining methods were largely mechanized. Between the two world wars, 500 workers were still busy with the dismantling. Since then, however, production has declined sharply.

use

Woman sculpture from the Cimetière des Carmes cemetery in Clermont-Ferrand

The material properties of the Volvic stone make it an excellent stone . As early as the 12th and 13th centuries it was used to build churches in the area around Clermont-Ferrand and then spread from here to other cities in the Basse Auvergne such as Riom. In addition to its use for sacred buildings, the stone found more and more entry into the secular area of ​​the cities and gave them a special note because of its dark appearance. Examples of secular architecture are the Hôtel Savaron from 1513 or the Fontaine d'Amboise fountain from 1515, both in Clermont-Ferrand.

The Volvic stone owed a new upswing from 1812 to Gaspard de Chabrol , a polytechnician and prefect of the Seine department . He had numerous public roads and paths construction carried out, with the Volvic stone being used for pavement borders and other uses.

Because of its texture and hardness, the Volvic stone is an ideal material for sculptures . It was also used in numerous grave monuments. Good examples of sculptures made from Volvic stone can be seen on the Chemin Fais'Art , a circular art trail near Chapdes-Beaufort .

Its good temperature resistance and its high melting point (around 1500 ° C) allow the stone to be coated with enamel , which happens at an average temperature of 900 ° C. This process was initiated by Gaspard de Chabrol. As prefect of the Seine département between 1812 and 1815, he had enamelled street signs set up in Paris for the first time. His initiative was then legally immortalized in 1844 by a decree of the Prefect Claude-Philibert Barthelot de Rambuteau .

Michelin road sign in the Deux-Sèvres department

The tire manufacturer Michelin also used the Volvic stone for place and street signs.

The consumption of Volvic stone has meanwhile decreased, but the stone is still used in the arts and crafts and especially for panorama boards.

Buildings with Volvic stone

The Saint-Amable basilica in Riom

The Volvic stone was first mentioned in writing in 1254. However, it was used almost a century earlier in Riom for the construction of Saint-Amable. The oldest known building made of Volvic stone is the Notre-Dame church in Herment from 1145, but Notre-Dame-de-Port in Clermont is probably a bit older.

Sacred buildings

The following church buildings used Volvic stone:

Profane architecture

Maison de l'Éléphant in Montferrand

Examples of secular architecture with Volvic stone are:

Individual evidence

  1. ↑ Yves ConnieR and Marc Prival: Volvic: Une pierre et des hommes . Éditions Créer, Saint-Just-près-Brioude 2008.
  2. ↑ René C. Maury, Robert Brousse, Benoît Villemant, Jean-Louis Joron, Henri Jaffrezic et Michel Treuil: Cristallisation fractionnée d'un magma basaltique alcalin: la série de la Chaîne des Puys (Massif Central, France) . In: Bull. Minéral . tape 103 , 1980, I. Pétrologie, p. 250–266 ( online - PDF; 2.3 MB ).
  3. ^ G. Guérin: Thermoluminescence des plagioclases. Méthode de datation du volcanisme. Applications au domaine volcanique français: Chaîne des Puys, Mont Dore et Cézallier, Bas Vivarais. Thèse d'Etat . Université Pierre et Marie Curie, Paris 1983, p. 253 .
  4. ^ D. Morel and J. Picot: La couleur de la ville médiévale. Matériaux et identité urbaine des centres politiques d'Auvergne (XIIe - XVe siècle) . In: Construire la ville . Éditions du CTHS, Paris 2014, p. 141-153 .